1. Field of the Invention
The present invention relates in general to a warning system of a motor vehicle, and more particularly to a so-called "oil degradation warning system" which automatically gives a visual or audible warning when the engine oil has been degraded to a certain unusable degree.
2. Description of the Prior Art
As is known, engine oil functions not only to lubricate movable metal parts, such as pistons or the like, but also to remove heat from heated parts of the engine. However, long use of the engine oil degrades the nature thereof and thus prevents the oil from exhibiting satisfied lubrication and heat removing functions.
In view of the above, several kinds of oil degradation warning systems have been hitherto proposed, which, by using intake vacuum, oil temperature, and engine speed and/or moved distance as parameters for estimating the degradation degree of the engine oil, calculate the time (viz., the oil change time) when the oil soould be changed with new or fresh one and give a visual or audible warning at that time. Some of them are disclosed in Japanese Patent Application First Provisional Publications Nos. 59-27260 and 59-43299.
However, in these hitherto proposed systems, the parameters as mentioned hereinabove are "totally" treated for calculating the oil change time, so that it inevitably occurs that the oil change time is calculated with noticeable error. Thus, in such systems, it sometimes occurs that the warning is given considerably before or after the time when the oil change is practically necessary.